Two-wire web formers (gap formers) are generally divided in two basic types, namely roll gap formers and blade gap formers.
In a roll gap former, the pulp stock is delivered from a head box into a gap formed by a forming roll and two wires, whereafter the wires and the pulp stock therebetween follow the curvature of a forming roll for removing most of the water through the wires. The necessary dewatering pressure is provided by the tautness of an outer wire for generating in the stock a pressure which is directly proportional to the wire tautness and inversely proportional to the radius of curvature of the forming roll. In the gap, some of the kinetic energy of a jet coming from the head box converts into pressure energy and the jet speed is reduced accordingly. Dewatering away from the forming roll is intensified by centrifugal force. Towards the forming roll, dewatering can be intensified by means of vacuum. In this case, the forming roll must be constructed as a vacuum roll.
Typically of a roll gap former, the produced paper has a rather poor formation (a small-scale surface weight dispersion) and a good retention or the ratio of the amount of solids retained in a web to be formed to the amount of solids discharged from the head box.
The other basic design of a two-wire former is a so-called blade gap former which is characterized in that a slice jet discharging from the head box is delivered into a gap which is formed by two wires and converges either into a substantially straight-lined dewatering zone, formed by means of dewatering blades located on either side of the wires transversely to the traveling direction of the wires, or into one or two successive curved dewatering zones, wherein the blades are located towards the center of curvature of the wires. This type of formers are disclosed, for example, in U.S. Pat. No. 3,578,558, German Publication print 21 13 014, U.S. Pat. No. 3,944,464 and U.S. Pat. No. 4,125,428, and Finnish 1.0 Publication print 50647.
In turn, a blade gap former has typically a good formation and a low retention.
The above basic types can be combined, for example, in such a manner that the forming zone commences as a roll gap former and continues as a former, in which the blades are positioned the same way as in a blade gap former, whereby its qualities are determined on the basis of the relative proportion of the above-described dewatering techniques. This type of solutions are disclosed, for example, in Finnish Publication print 83102 and Finnish Patent 77702. Despite the efforts of combining these former types, the poor qualities of both basic types shall partially remain.
An object of this invention is to introduce a two-wire web former which does not possess the above-described drawbacks found in the basic types. The invention is capable of providing a gap supported by an open breast roll, wherein the dewatering is nevertheless not excessive prior to location of a guide element converging the wires at which the-dewatering is allowed to continue. This is of major importance especially for improving of the formation.